CVE-2026-56865
High
Description
A malicious GOPROXY was previously capable of forging up to two sumdb tiles that allow for a requested module to bypass the GOSUMDB check and persist attacker-controlled module content to a local Go module cache. This attack allows for a malicious GOPROXY to serve malicious module content that cannot be detected by evaluating the transparency log. All tiles are now correctly verified against their parents. In order to determine if you have been affected: rm -r go.sum go.work.sum vendor/ && go mod tidy
SeverityHigh
CVSS8.4
Exploit probability—
ProductToolchain
PublishedAug 13, 2026
First seenAug 13, 2026
Weakness typeCWE-347CWE-347
How to fix
Update
- toolchain1.25.13, 1.26.6, 1.27.0-rc.3 or later
- golang.org/x/mod0.40.0 or later
Affected products
| Product | Affected versions | Fixed in |
|---|---|---|
| toolchainGo | v < 1.25.13, 1.26.0-0 ≤ v < 1.26.6, 1.27.0-0 ≤ v < 1.27.0-rc.3 | 1.25.13, 1.26.6, 1.27.0-rc.3 |
| golang.org/x/modGo | v < 0.40.0 | 0.40.0 |
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